Optical registration medium with dual information layer

ABSTRACT

A read-only optical registration medium which successively includes: 
     a transparent substrate; 
     a first registration surface containing localized level variations representing binary data bits; 
     a protective layer, 
     whereby: 
     between the substrate and the first registration surface, the medium further includes a dielectric trilayer which contains a first, second and third dielectric layer having respective refractive indices n 1 , n 2  and n 3 , whereby, at both a first wavelength λ 1  and a second wavelength λ 2  : 
     n 2  &lt;n 1  and n 2  &lt;n 3 , the trilayer being constituted so as to be transparent at λ 1  and to be semi-reflective at λ 2  ; 
     the surface of the trilayer nearest the substrate contains localized level variations representing binary data bits, thereby forming a second registration surface. 
     An alternative registration medium according to the invention successively includes: 
     a substrate (4); 
     a first registration surface containing localized level variations representing binary data bits; 
     a transparent protective layer, 
     whereby: 
     between the protective layer and the first registration surface, the medium further includes a dielectric trilayer which contains a first, second and third dielectric layer having respective refractive indices n 1 , n 2  and n 3 , whereby, at both a first wavelength λ 1  and a second wavelength λ 2  : 
     n 2  &lt;n 1  and n 2  &lt;n 3 , 
     the trilayer being constituted so as to be transparent at λ 1  and to be semi-reflective at λ 2  ; 
     the surface of the trilayer nearest the protective layer contains Iocalized level variations representing binary data bits, thereby forming a second registration surface.

BACKGROUND OF THE INVENTION

The invention relates to a read-only optical registration medium whichsuccessively comprises:

a substrate;

a first registration surface containing localised level variationsrepresenting binary data bits;

a protective layer, whereby the substrate and/or the protective layerare transparent.

Such an optical registration medium can be read by scanning a focusedlight beam along its registration surface, and monitoring the light beamreflected therefrom. Because of the localised level variations in theregistration surface, the phase of the reflected light beam will dependon the location of the point from which it is reflected, assuming theamplitude of the level-variations to be smaller than the coherencylength of the incident light beam. Using interference effects, it thusbecomes possible to optically read the topography of the registrationsurface. Depending on which of the two is transparent (at the wavelengthλ of the incident light beam), such scanning can be performed througheither the substrate or the protective layer.

An example of a well-known optical registration medium as specified inthe opening paragraph is the Compact Disc (CD), which can be embodied invarious forms (e.g. CD-audio, CD-ROM, CD-interactive and photo-CD), allof which consist of one-sided polycarbonate discs having an embossedregistration surface which is metallised and overlaid by a thintransparent resin layer. In these media, the localised level variationsin the reflective registration surface are embodied as sharply-defined,micron-sized pits in an otherwise level plane. The depth of these pitsis chosen to have a value λ/4n, where n is the refractive index of themedium through which the scanning light beam is incident (conventionallythe substrate). As a result, a light beam reflected from the bottom of apit will demonstrate a path-difference of λ/2n (phase-difference of π)with respect to a light beam reflected from the surface of thesurrounding plane.

A significant drawback of the present CD is that it does not havesufficient storage capacity to accommodate an average movie (both soundand vision) on its single registration surface. As a result, althoughthe CD has rendered the gramophone record virtually defunct, and hasalso substantially replaced the Compact Cassette, it has not yet beenable to seriously compete with the VHS video tape as a commercialaudio/video carrier.

One means of addressing this capacity problem is discussed in EuropeanPatent Application EP 0 635 825, which describes a medium having stackedregistration surfaces. These surfaces are optically accessed from asingle side of the disc, by shifting the focal plane of the incidentlight beam to any of a number of discrete positions corresponding to thelocations of the various registration surfaces. However, this approachis plagued by considerable focusing-error problems, whereby unwantedreflected light from a foreground or background registration surface canconfuse the tracking optics in a playback device, causing it to lose itsfocus on the registration surface being read at that time. In addition,the need to focus discretely at various depths in the disc requires theuse of complicated mechanics and servo-electronics in the read "head".

An alternative to the approach in the preceding paragraph is to make theCD double-sided, i.e. to provide a registration surface upon each majorface of its substrate disc. However, such an approach has a number ofsignificant disadvantages. For example:

Assuming that the two registration surfaces are not individuallyaccessed by means of a focal-plane shift (as in the previous paragraph),then each registration surface must be read via the immediatelyoverlying (thin) protective layer instead of via the underlying (thick)substrate (which no longer has to be transparent). As a result, dust orscratches on the surface of the disc will produce far greater noise thanin the case of substrate-incident reading, since such unwanted surfacialfeatures eclipse a greater fraction of the focusing cone as theirproximity to the focal point (i.e. the registration surface) increases;Because such a disc is double-sided, it must either be turned over assoon as the first registration surface has been read (which necessitatesan undesirable playback interruption) or it must be read using aplay-back device which positions a focused light beam at each side ofthe disc plane (thereby causing an increase in the physical size of theplay-back device). Only in the second scenario can both registrationsurfaces be read simultaneously; Since optical access is required fromboth sides of the disc, an identification label cannot be applied to oneof the disc faces.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an optical registrationmedium having increased storage capacity compared to correspondingstate-of-the-art media. Specifically, it is an object of the inventionto produce an optical registration medium having dual registrationsurfaces. Moreover, it is an object of the invention that it should bepossible to read both registration surfaces in such a medium from asingle side. In particular, it is an object of the invention that thenovel medium should at least be capable of accommodating a movie (audioand video content) having a duration of two hours.

These and other objects are achieved according to the invention in twodistinct media, namely:

(1) A read-only optical registration medium which successivelycomprises:

a transparent substrate;

a first registration surface containing localised level variationsrepresenting binary data bits;

a protective layer,

characterised in that:

between the substrate and the first registration surface, the mediumfurther comprises a dielectric trilayer which contains a first, secondand third dielectric layer having respective refractive indices n₁, n₂and n₃, whereby, at both a first wavelength λ₁ and a second wavelengthλ₂ :

n₂ <n₁ and n₂ <n₃,

the trilayer being constituted so as to be transparent at λ₁ and to besemi-reflective at λ₂ ;

the surface of the trilayer nearest the substrate contains localisedlevel variations representing binary data bits, thereby forming a secondregistration surface.

(2) A read-only optical registration medium which successivelycomprises:

a substrate;

a first registration surface containing localised level variationsrepresenting binary data bits;

a transparent protective layer,

characterised in that:

between the protective layer and the first registration surface, themedium further comprises a dielectric trilayer which contains a first,second and third dielectric layer having respective refractive indicesn₁, n₂ and n₃, whereby, at both a first wavelength λ₁ and a secondwavelength λ₂ :

n₂ <n₁ and n₂ <n₃,

the trilayer being constituted so as to be transparent at λ₁ and to besemi-reflective at λ₂ ;

the surface of the trilayer nearest the protective layer containslocalised level variations representing binary data bits, therebyforming a second registration surface.

The term "transparent" should here be interpreted as indicating atransmission of at least 85%, and preferably at least 90%, whereas theterm "semi-reflective" should here be interpreted as indicating areflection of about 25-40%. Medium (1) can be read from the substrateside, and medium (2) can be read from the side of the protective layer.

The dielectric trilayer in the inventive medium is thus constituted thatit acts as a window at λ₁ and partially as a mirror at λ₂. Consequently,light directed towards the first registration surface via the dielectrictrilayer can follow two different paths, depending upon its wavelengthλ:

If λ=λ₁, then the incident light beam will penetrate beyond the (nearer)second registration surface, and will be reflected from the (moreremote) first registration surface;

If λ=λ₂, then the incident light beam will be reflected from bothregistration surfaces, though at generally different intensities.

The mechanism of the invention as described in the previous paragraphrelies on the fact that the respective Reflection Coefficients R₁, R₂and Transmission Coefficients T₁, T₂ at the first and secondregistration surfaces are functions of wavelength, and demonstratesubstantially different values at the wavelengths λ₁ and λ₂. Assuming anincident light beam to have an intensity I (and to enter from that sideof the medium to which the second registration surface is closest), then(with reference to FIGS. 1 and 2) the intensity of the reflected beamfrom the first registration surface will be:

    I.sub.1 (λ)=IT.sub.2 R.sub.1 T.sub.2 =IR.sub.1 T.sub.2.sup.2,

and the intensity of the reflected beam from the second registrationsurface will be:

    I.sub.2 (λ)=IR.sub.2.

It will be specifically demonstrated in a particular embodimentherebelow that I₁ and I₂ can have drastically different values at λ₁ andλ₂.

The employed thicknesses of the individual layers in the dielectrictrilayer (hereinafter respectively denoted by t₁, t₂ and t₃) will dependon the particular values of n₁, n₂ and n₃ in a given embodiment, as wellas on the desired values of λ₁ and λ₂. In general, each of thesethicknesses will have a value of the order of 10-100 nm, though smalleror larger values are also possible.

Ideally, all three constituent layers in the inventive medium'sdielectric trilayer have an absorption coefficient k=0 (k being theimaginary part of the complex refractive index n=n+ik). In practice,however, this condition cannot be perfectly achieved, and one shouldtherefore endeavour to achieve a value of k which is as small aspossible.

The dual registration surfaces in the inventive optical registrationmedium may be read either simultaneously or consecutively. In a specificapplication of the former method, one of the dual registration surfacescan be encoded with audio information and the other can be encoded withvideo information, so that simultaneous play results in a motion picturesuch as a movie, pop video, television program, etc.

It should be explicitly noted that the substrate referred to in thecontext of the current invention may have various forms, and may inparticular be embodied as a flexible or rigid disc (e.g. ofpolycarbonate), or as a flexible elongated tape (e.g. of roughenedpolyethene terephthalate). In addition, although the invention providesdual registration surfaces which can be read from a single side of thesaid substrate, this does not preclude the occurrence of yet anotherregistration surface (or surfaces) at the other side of the substrate.In particular, one can envisage a double-sided disc which has aninventive dual registration surface on each side; such a disc can beread on either side via its protective layer (type (2) medium).

A preferential embodiment of the inventive registration medium ischaracterised in that, at λ₁ =780 nm and at λ₂ =630 nm:

2.5≦n₁ ≦3.5;

1.5≦n₂ ≦2.0;

2.5≦n₃ ≦3.5.

The quoted value of λ₁ is the current CD standard wavelength, whereasthe stated value of λ₂ is the recently agreed supplementary CD standardwavelength. These wavelengths can be obtained from lasers based on therespective material systems GaAs/AlGaAs and GaAs/InGaP/InAlGaP, forexample. An immediate advantage of such an embodiment is itsbackwards-compatibility: the first registration surface can be read by aconventional CD player (λ=780 nm), whereas both registration surfacescan be read by a new-style player employing two laser wavelengths.

An advantageous embodiment of the inventive registration medium ischaracterised in that the first and third dielectric layers comprise amaterial selected from the group consisting of zirconium nitride,silicon carbide and zirconium aluminium nitride, and that the seconddielectric layer comprises a material selected from the group consistingof silicon oxide, silicon nitride, yttrium oxide, aluminium oxide andaluminium nitride. It should be noted that, in the case of multivalentelements, the terms "nitride", "carbide" and "oxide" as here employedare intended to encompass the various compounds which can result fromsuch multiple valency: for example, "silicon oxide" should be seen asreferring to either SiO₂ or sub-oxides like SiO. All of the listedmaterials can be readily provided using, for example, sputterdeposition, physical vapour deposition or laser ablation deposition (allin a reactive gaseous atmosphere), or by chemical vapour deposition. Inaddition, their refractive indices all fall within the ranges stipulatedin the previous paragraph, and their absorption coefficients arenegligible.

As an alternative to the above-mentioned embodiment, it is alsopossible, for example, to use various dyes in the first and/or thirddielectric layers, so as to achieve a relatively large n-value togetherwith a relatively small k-value. Such a dye can, for example, bedissolved in an organic resin, which can then be applied in the form ofa layer by spin-coating, spraying, roller-coating, dip-coating, etc.,before being subsequently hardened (e.g. by curing or drying). Aspecific example of a suitable such dye isbutyl-benzo-indo-carbocyanine, for which n=3.050 and k=0.058 (at 780nm).

It should be explicitly noted that the first and third dielectric layersdo not have to have the same material constitution.

In a particular embodiment of the inventive registration mediuminvestigated by the inventors, a transparent polycarbonate substratedisc was overlaid by a dielectric trilayer consisting of sputtered ZrN,AlN and ZrN layers. These layers were respectively characterised by thevalues:

    t.sub.1 =t.sub.3 ≈26 nm, t.sub.2 ≈97 nm, n.sub.1 =n.sub.3 ≈3, n.sub.2 ≈2.

The said trilayer was itself overlaid by a layer of cured polyacrylateresin, whose exposed surface was metallised with a thin film of Al. Forlight incident via the substrate, it was found that:

At λ₁ =780 nm:

R₁ ≈0.75, R₂ ≈0, T₂ ≈1.0, so that I₁ ≈0.75I and I₂ ≈0;

At λ₂ =630 nm:

R₁ =R₂ ≈0.3, T₂ ≈0.7, so that I₁ ≈0.151 and I₂ ≈0.3I.

In this case, it is obvious that, at 630 nm, both the first and thesecond registration surfaces can be read, but that the reflectedintensity from the second registration surface is twice as great as thatfrom the first registration surface. This substantial difference inintensity reduces the risk of occurrence of tracking errors as a resultof unwanted background reflections (from the first registrationsurface).

A particular embodiment of the inventive medium is characterised in thatthe first registration surface is metallised, i.e. provided with ametallic reflection layer. Such a reflection layer may comprise a puremetal or an alloy, and may be provided using sputter deposition, vapourdeposition or laser ablation deposition, for example. Exemplarymaterials for this purpose include aluminium, gold, copper, silver, andtheir alloys. Use of such a reflection layer ensures that, when theregistration surface is scanned with a focused light beam, enough lightintensity is reflected to yield an acceptable output signal level.

An alternative embodiment of the inventive medium is characterised inthat use is made of at least one dye to ensure that the (complex)refractive index of the material at one side of the first registrationsurface has a substantially different value to the (complex) refractiveindex of the material at the other side of the first registrationsurface (at λ=λ₁, at least). In this manner, it is possible tosubstantially increase the value of R₁. In particular, for a sufficientdifference in refractive index, Total Internal Reflection can be inducedat this interface, with a relatively small associated Brewster angle.The said difference in complex refractive index n may lie either in n orin k, or both. However, in the case of a dye employed at thelight-incident side of the first registration surface, the value of kfor that dye should be as small as possible.

As a specific example of the embodiment just discussed, reference ismade to a medium in which the first registration surface is embossed ina layer L_(a) of a polyacrylate resin, and is then overlaid by aprotective layer L_(b) of a polyurethane resin. It is assumed that thismedium is to be read via its substrate, which implies that L_(a) is atthe light-incident side of the first registration surface. In such acase, one may, for example, incorporate a pyrylium-4,4'-cyanine dye(n=3.270, k=0.570 at 780 nm) in the layer L_(b), or the aforementionedbutyl-benzo-indo-carbocyanine dye (n=3.050, k=0.058 at 780 nm) in thelayer L_(a).

It should be explicitly noted that the registration medium according tothe invention may comprise various other layers in addition to thosereferred to heretofore. For example, an adhesion-promoting layer (ofzirconium oxide, for example) may be employed on the embossed surface onwhich the dielectric trilayer is provided. Alternatively, additionaldielectric layers may be employed in conjunction with the saiddielectric trilayer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and its attendant advantages will be further elucidatedwith the aid of exemplary embodiments and the accompanying schematicdrawings (not to scale), whereby:

FIG. 1 renders a cross-sectional view of part of a particular embodimentof an optical registration medium according to claim 1 of the invention;

FIG. 2 renders a cross-sectional view of part of a particular embodimentof an optical registration medium according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Embodiment 1

FIG. 1 is a cross-sectional depiction of part of an optical registrationmedium 2 according to claim 1 of the invention. A substrate 4 isembossed on one side so as to form a surface 6 containing localisedlevel variations which represent binary data bits. For example, suchvariations may be embodied as a pattern of pits in an otherwise levelsurface, these pits being of uniform depth (e.g. 150 nm) and width (e.g.500 nm), but having variable length.

A dielectric trilayer 8 has been deposited directly on the surface 6.This trilayer 8 comprises first, second and third dielectric layers 81,82, 83 having respective thicknesses t₁, t₂, t₃ and refractive indicesn₁, n₂, n₃. The materials of the layers 81, 82, 83 are thus chosen that,at both λ₁ =780 nm and λ₂ =630 nm:

n₂ <n₁ and n₂ <n₃.

In combination with these refractive indices, the values of t₁, t₂ andt₃ are thus chosen that the trilayer 8 is transparent at λ₁ andsemi-reflective at λ₂. The surface 6 corresponds to the "secondregistration surface" S₂ referred to in claim 1.

The trilayer 8 is overlaid by a transparent layer 10 whose upper surfacehas been embossed so as to form a surface 12 containing localised levelvariations which represent binary data bits (e.g. embodied as pits suchas those described hereabove). This surface 12 has been metallised witha thin reflective film 14, which is overlaid by a protective layer 16.The surface 12 corresponds to the "first registration surface" S₁referred to in claim 1.

The Figure also depicts a light beam 18 which is incident on the surfaceS₂ via the substrate 4. Such a beam 18 will, in general, undergo partialreflection at the surface S₂. If the incident intensity of the beam 18is I, then the intensity of the beam 20 reflected from the surface S₂will be IR₂ (=I₂).

The beam 18 will also, in general, undergo partial penetration of thesurface S₂, with an intensity IT₂. This penetrative beam will undergoreflection at the surface S₁, and the reflected beam which thus ariseswill emerge through the substrate 4 as a beam 22 of intensity IR₁ T₂ ²(=I₁).

As already discussed hereabove, the values of I₁ and I₂ depend stronglyon the wavelength of the incident beam 18.

Embodiment 2

FIG. 2 shows a cross-section of part of a particular embodiment of anoptical registration medium in accordance with claim 2 of the invention.Those features in FIG. 2 which correspond to given features in FIG. 1have been given the same reference labels.

The medium 2 comprises a substrate 4 which has been embossed on one sideso as to form a surface 6 containing localised level variations whichrepresent binary data bits (e.g. in the form of the pits referred to inEmbodiment 1). This surface 6 is metallised with a thin reflective film14 so as to form the "first registration surface" S₁ referred to inclaim 2.

The layer 14 is overlaid by a transparent layer 10 whose upper surfacehas been embossed so as to form a surface 12 containing localised levelvariations which represent binary data bits (e.g. in the form of thesaid pits). A dielectric trilayer 8 has been deposited directly on thissurface 12. This trilayer 8 comprises first, second and third dielectriclayers 81, 82, 83 having respective thicknesses t₁, t₂, t₃ andrefractive indices n₁, n₂, n₃. The materials of the layers 81, 82, 83are thus chosen that, at both λ₁ =780 nm and λ₂ =630 nm:

n₂ <n₁ and n₂ <n₃.

In combination with these refractive indices, the values of t₁, t₂ andt₃ are thus chosen that the trilayer 8 is transparent at λ₁ andsemi-reflective at λ₂. The surface of the trilayer 8 remote from thesubstrate 4 corresponds to the "second registration surface" S₂ referredto in claim 2. This surface S₂ is overlaid by a protective layer 16.

FIG. 2 also shows a light beam 18 which is incident on the surface S₂via the protective layer 16. Such a beam 18 will, in general, undergopartial reflection at the surface S₂. If the incident intensity of thebeam 18 is I, then the intensity of the beam 20 reflected from thesurface S₂ will be IR₂ (=I₂).

The beam 18 will also, in general, undergo partial penetration of thesurface S₂, with an intensity IT₂. This penetrative beam will undergoreflection at the surface S₁, and the reflected beam which thus ariseswill emerge through the protective layer 16 as a beam 22 of intensityIR₁ T₂ ² (=I₁).

Once again, the values of I₁ and I₂ depend strongly on the wavelength ofthe incident beam 18.

Embodiment 3

With reference to FIGS. 1 and 2, the various labelled constituents mayhave the following exemplary specifications:

4: transparent polycarbonate disc, diameter 12 cm and thickness 1.2 mm.An alternative is glass. A thickness of 0.6 mm is also conceivable infuture discs;

81: sputter-deposited ZrN, t₁ =23-29 nm. Alternatives include ZrAlN andSiC_(x) ;

82: sputter-deposited AlN, t₂ =94-100 nm. Alternatives include SiO, SiN,Y₂ O₃ and Al₂ O₃ ;

83: same as 81 (t₃ =t₁);

10: spin-coated, UV-cured transparent polyacrylate resin layer,thickness 20-40 μm.

Alternatives include transparent polyurethane resins;

14: sputter-deposited aluminium, thickness 50-80 nm. Alternativesinclude Au, Ag, and Cu;

16: similar to 10, but with a thickness of 3-10 μm. Alternatives includetransparent inorganic coatings such as SiO₂ and Si₃ N₄ (thickness100-500 nm).

Embodiment 4

In the case of the medium depicted in FIG. 2, the substrate 4 may beembodied as a flexible longitudinal tape. A suitable material for thispurpose is, for example, polyethene terephthalate (PET) containingdispersed microscopic fillers (e.g. SiO₂ particles with a diameter of400 nm) or having a rough backing layer (e.g. 300-nm chromium trioxideparticles embedded in a resin binder. More information with regard tothe fundamental constitution of such an optical registration tape can begleaned from non-prepublished European Patent Application No. 95202686.2(PHN 15.500).

Embodiment 5

In a medium as discussed in Embodiment 3, the layers 81 and 83 eachcomprise silicon carbide (SiC_(x)) with a refractive index n₁ =n₃ ≈3,and the layer 82 comprises silicon carbide with a refractive index n₂≈2. All three layers 81,82,83 were sputter-deposited in a reactivehydrocarbon atmosphere from a Si sputter block. The composition of thereactive atmosphere determined the refractive index of the resultinglayer.

In a specific example, the reactive atmosphere comprised a mixture of C₂H₂ and Ar. The sputter target was a Si plate with a face area of 127×445mm², and the sputter power was 2.5 kW. The composition of the reactiveatmosphere was as follows:

    ______________________________________                                        For n.sub.1 = n.sub.3 ≈ 3:                                                           Ar flow:     120    sccm                                                      C.sub.2 H.sub.2 flow:                                                                      12.5   sccm                                       For n.sub.2 ≈ 2:                                                                     Ar flow:     120    sccm                                                      C.sub.2 H.sub.2 flow:                                                                      20     sccm                                       ______________________________________                                    

As an alternative to silicon carbide, the layer 82 could have comprisedSiN, for example.

We claim:
 1. A read-only optical registration medium which successivelycomprises:a transparent substrate; a first registration surfacecontaining localised level variations representing binary data bits; aprotective layer, characterised in that: between the substrate and thefirst registration surface, the medium further comprises a dielectrictrilayer which contains a first, second and third dielectric layerhaving respective refractive indices n₁, n₂ and n₃, whereby, at both afirst wavelength λ₁ and a second wavelength λ₂ : n₂ <n₁ and n₂ <n₃, thetrilayer being constituted so as to be transparent at λ₁ and to besemi-reflective at λ₂ ; the surface of the trilayer nearest thesubstrate contains localised level variations representing binary databits, thereby forming a second registration surface.
 2. A read-onlyoptical registration medium which successively comprises:a substrate; afirst registration surface containing localised level variationsrepresenting binary data bits; a transparent protective layer,characterised in that: between the protective layer and the firstregistration surface, the medium further comprises a dielectric trilayerwhich contains a first, second and third dielectric layer havingrespective refractive indices n₁, n₂ and n₃, whereby, at both a firstwavelength λ₁ and a second wavelength λ₂ : n₂ <n₁ and n₂ <n₃, thetrilayer being constituted so as to be transparent at λ₁ and to besemi-reflective at λ₂ ; the surface of the trilayer nearest theprotective layer contains localised level variations representing binarydata bits, thereby forming a second registration surface.
 3. An opticalregistration medium according to claim 1, characterised in that, at λ₁=780 nm and at λ₂ =630 nm:2.5≦n₁ ≦3.5; 1.5≦n₂ ≦2.0; 2.5≦n₃ ≦3.5.
 4. Anoptical registration medium according to claim 1, characterised in thatthe first and third dielectric layers comprise a material selected fromthe group consisting of zirconium nitride, silicon carbide and zirconiumaluminium nitride, and that the second dielectric layer comprises amaterial selected from the group consisting of silicon oxide, siliconnitride, yttrium oxide, aluminium oxide and aluminium nitride.
 5. Anoptical registration medium according to claim 1, characterised in thatthe first registration surface is metallised.
 6. An optical registrationmedium according to claim 1, characterised in that the refractive indexof the material at one side of the first registration surface has asubstantially different value to the refractive index of the material atthe other side of the first registration surface.
 7. An opticalregistration medium according to claim 1, characterised in that thesubstrate has the form of a disc.
 8. An optical registration mediumaccording to claim 1, characterised in that the substrate has the formof a flexible, elongated tape.
 9. An optical registration mediumaccording to claim 2, characterised in that, at λ₁ =780 nm and at λ₂=630 nm:2.5≦n₁ ≦3.5; 1.5≦n₂ ≦2.0; 2.5≦n₃ ≦3.5.
 10. An opticalregistration medium according to claim 2, characterised in that thefirst and third dielectric layers comprise a material selected from thegroup consisting of zirconium nitride, silicon carbide and zirconiumaluminum nitride, and that the second dielectric layer comprises amaterial selected from the group consisting of silicon oxide, siliconnitride, yttrium oxide, aluminum oxide and aluminium nitride.
 11. Anoptical registration medium according to claim 2, characterised in thatthe first registration surface is metallised.
 12. An opticalregistration medium according to claim 2, characterised in that therefractive index of the material at one side of the first registrationsurface has a substantially different value to the refractive index ofthe material at the other side of the first registration surface.
 13. Anoptical registration medium according to claim 2, characterised in thatthe substrate has the form of a disc.
 14. An optical registration mediumaccording to claim 2, characterised in that the substrate has the formof a flexible, elongated tape.